The invention relates to a pulse compression radar of the almost linearly frequency-modulated type of the transmitted signal during the pulse, comprising a transmit circuit including a high-frequency oscillator/amplifier, a transmit-receive aerial and a receive circuit having a pulse compression element.
The pulse compression radars have a multitude of applications. They operate for the greater part in the X-band and can be arranged on the ground or be airborne. In the latter case, they are specifically used for maritime surveillance, mapping and meteorology.
In these radars, travelling-wave tubes are currently used as high-frequency oscillator-amplifiers in the transmit circuits, that is to say as power sources. It is likewise possible to use solid state Impatt diode power sources.
The principle of the pulse compression radars is widely known and described, for example, in "Technique de l'ing enieur, Electronique", Vol. 5, E 6660-1, 24. Compared to the conventional pulse radar, the pulse compression radar allows to maintain the independence between the duration T and the band .DELTA.F of the signal, which leads to the compression coefficient: T..DELTA.F. From a practical point of view this means that at an equal power level of the two aforementioned types of radar, the range and definition of the pulse compression radar are better as regards the compression coefficient compared to the pulse radar having the same power level. This fundamental advantage is gained by means of a complication of the radar system. With conventional power amplifying pulse radar, a filter .gamma.(f) shifting the phase of the various frequencies of the spectrum of the transmitted signal in different ways, is to be inserted after the slicer of the intermediate-frequency transmission signal, and at the receive end, after the frequency change, a filter .gamma.*(f) which is to compensate for the phase-shifts caused by the filter .gamma.(f). All the practical methods narrow down to using, for the pulse expansion a linear or almost linear modulation of the frequency of the transmitted signal, followed by a reversed linear frequency modulation at the receive end. It should be observed that the transmit circuit obtained thus is complex because it comprises, upstream of the microwave-frequency oscillator/amplifier, the following cascaded elements: a first local oscillator OL1 which produces the intermediate frequency, a slicer for shaping the pulses transmitted at intermediate frequency, the filter (f) always operating at intermediate frequency and a mixer which receives at a second input the signal from a second local oscillator OL2. Besides, if one wishes to modify the characteristics of the prior art pulse compression radar, the two filters .gamma.(f) and .gamma.*(f) which have to remain adapted to each other are to be changed. Various techniques have in turns been used to manufacture the filters .gamma.(f) and .gamma.*(f): distributed filters, acoustic filters, bulk wave quartz lines, more recently ground-wave comb-shaped transducer lines or analog sampled lines. To produce the low-power high-frequency pulse signal, it is not always necessary to use a dispersive line for the filter .gamma.(f), because it can also be produced directly, just like an oscillator command. However, even in the latter case, the oscillator operates at intermediate frequency and the obtained system remains complex and thus expensive.